The present invention generally relates to plugging and abandonment of oil and gas wells. More particularly, the present invention relates to the removal of a tubular from a wellbore in order to satisfy various environmental regulations. More particularly still, the invention relates to severing nested strings of tubulars that are cemented together in order to more easily handle the tubulars as they are removed from a wellbore during or subsequent to a plugging and abandonment operation.
In the completion of oil and gas wells, boreholes are formed in the earth and thereafter are lined with steel pipe known as casing. An annular area formed between the outside of the casing and the wall of the borehole is typically filled with cement in order to secure the casing in the borehole and to facilitate the isolation of certain areas of the wellbore for the collection of hydrocarbons. In most instances, because of the depth of a wellbore, concentric strings of tubulars are disposed in the wellbore with each lower string of tubulars being necessarily smaller in diameter than the previous string. In some cases, especially in offshore oil and gas wells, the strings are run in a nested fashion from the surface of the well. In other words, a first string of casing is cemented into the wellbore and, subsequently, a second smaller string of casing is cemented into the first string to permit the borehole to be lined to a greater depth. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
When a decision is made to no longer operate a hydrocarbon well, the wellbore is typically plugged to prevent formation fluids from migrating towards the surface of the well or into a different zone. Various environmental laws and regulations govern the plugging and abandonment of wellbores. These regulations typically require that the wellbore be filled with some amount of cement. In some instances, the cement must be squeezed into the annular area around the cemented casing in order to prevent fluids from migrating up towards the surface of the well on the outside of the casing through any cement gaps. In offshore wells, regulations typically require not only the foregoing steps, but also that a certain amount of wellbore casing be completely removed from the wellbore. For example, in some instances, the upper 1,000 feet of casing extending downward from the ocean floor into the wellbore must be removed to complete a plugging and abandonment operation.
Various methods and techniques have been developed and are currently utilized in order to remove casing from an offshore wellbore. Most often, some type of cutting device is run into the wellbore on a wireline or string of tubulars. The cutting device is actuated in order to sever the casing at a predetermined depth, creating separate upper and lower strings of casing. Thereafter, the upper string is pulled and brought to the surface.
Because of the great length and weight of the upper string of casing being removed, it is necessary to further sever the upper casing string as it is retrieved at the surface. Accordingly, the casing is further severed into predetermined lengths. This makes handling and disposal of the removed casing more efficient.
In some instances, the severed upper string of casing includes more than one set of tubulars. In other words, there is a first outer string of casing, and then a second smaller string of casing nested therein. In one example, the outer casing string is 13xe2x85x9c inches in diameter, and the smaller casing nested therein is 9⅝ inches in diameter. These two strings of severed casing will typically be joined by a layer of cement within the annular area. This cement layer adds to the weight of the severed casing string, making it even more desirable to cut the retrieved pipe into manageable sections.
A casing string is typically comprised of a series of joints that are 30 feet in length. The pipe joints are connected by threaded male-to-female connections. When retrieving a severed casing string during a plug and abandonment procedure, it is desirable to break the pipe string by unthreading the connected joints. However, this process is difficult where the severed string consists of outer and inner pipe strings cemented together. Further, there is little incentive to incur the time necessary to break the joints apart at the threads, as the pipe joints from an abandoned well will typically not be re-used. For these reasons, the severed casing is typically broken into smaller joints by cutting through the inner and outer strings at the surface of the well. The severed pipe sections are then recycled or otherwise disposed of.
In a conventional plug and abandonment operation, casing strings are severed generally as follows:
First, the casing string is severed within the wellbore. Typically, severance is accomplished at a depth of around 1,000 feet. Thereafter, the severed portion of casing is xe2x80x9cjackedxe2x80x9d out of the wellbore and raised to the surface of the rig platform using a platform-mounted elevator. As the upper end of the severed casing section reaches the floor of the platform, it is lifted to a predetermined height above a set of slips. The slips are then set, suspending the severed string of casing above the rig floor. A drilling machine then drills a hole completely through the casing, including any cement layer and smaller diameter casing which is cemented within the larger diameter casing. Thereafter, a pin or other retainer is inserted through the drilled hole to ensure that the smaller string of casing is anchored to the larger string. This method of drilling a hole through the casing and inserting a retainer pin is necessary to ensure that the smaller string of casing does not become dislodged from the larger string due to some failure of the cement layer there between.
After the inner casing string and cement therearound is anchored to the larger outer string, a band saw is used to cut the severed tubular into a predetermined length. The band saw operates with coolant to avoid the use of high temperature cutters or the production of sparks. Typically, a length of between fifteen and thirty feet is selected, with the cut being made above the retention pin. The newly severed, ten-foot portion of string is then transported to a barge or other transportation means for disposal or salvage.
With the slips disengaged, the elevator then raises the severed string of casing another length of approximately ten feet. The slips are then re-engaged and the drilling, anchoring and cutting procedure takes place again.
While the foregoing apparatus and method are adequate to dispose of strings of concentrically cemented casing, the operation necessarily requires personnel to be at the drilling mechanism and the band saw during the operation. The presence of personnel on a platform inherently carries risk. The risk is magnified when the personnel must be in close contact with the operating machinery.
There is a need, therefore, for a method and apparatus of disposing of concentric strings of tubular during a plugging and abandonment operation which does not require personnel to be located directly at the machinery performing the cutting operations. There is a further need for a method and apparatus which can be operated remotely by well platform personnel. There is yet a further need for an apparatus and method that can more safely and effectively sever strings of casing at a well site.
The present invention generally provides an apparatus and method for severing predetermined lengths of nested casing above a drilling rig or workover rig platform. The apparatus includes a clamp assembly, a drill assembly and a cutting assembly. In one aspect, the clamp assembly, the drilling assembly and the cutting assembly are disposed at the end of a telescopic arm, and are remotely operated by personnel using a control panel. In accordance with the present invention, the clamp assembly is positioned adjacent a section of casing to be severed, and then clamped thereto. Thereafter, the drilling assembly is actuated so as to drill a hole completely through the casing strings. A retention pin is then inserted through the newly formed aperture. Finally, the cutting assembly, such as a band saw, is actuated so as to severe the casing above the pin. The newly severed portion of casing above the pin may then be disposed of.